Puncture resistant heat shrinkable film containing narrow molecular weight ethylene alpha olefin

Abstract: A biaxially stretched film with very high heat shrink and puncture resistance properties, formed of low density ethylene alpha olefin copolymer having low melt index, low Mw/Mn, and melting point below about 102°C.

Description: RELATED APPLICATION This application is a continuation-in-part of Serial No. 08/089914 filed July 12, 1993 in the name of Paul N. Georgelos. FIELD OF THE INVENTION This invention relates to puncture resistant, heat shrinkable film, and more particularly to such films having high puncture resistance, high biaxial heat shrink and good optical properties. The film contains ethylene alpha olefin copolymer with high molecular weight but a relatively narrow distribution of molecular weights (low Mw/Mn). BACKGROUND OF THE INVENTION The shrink packaging of food articles such as poultry, fresh red meat and processed meat products requires tough, puncture resistant, yet flexible film materials which are heat shrinkable. Most commonly these films are fabricated as bags into which the food articles are placed. The food article-containing bag is then evacuated and sealed either by clips or more commonly by heat sealing the bag inner surfaces together across the bag's open mouth end. The sealed, food article-containing bag is then heated, for example by hot air, infrared radiation or hot water, causing the bag to shrink in both the machine direction (MD) and the transverse direction (TD) against the contours of the contained food article or articles. For many applications the heat shrunk package must have good optical properties, either for aesthetic reasons or functional reasons or both. In some instances the film must have low haze so that the packaged article may be periodically inspected to insure continued package integrity. For example, in commercial poultry shrink packaging operations, monolayer films formed from polyethylene or ethylene vinyl acetate polymers have been widely used. In general, polyethylenes such as linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) provide relatively high puncture resistance, high tensile strength, but relatively low biaxial shrink properties. On the other hand, ethylene vinyl acetate (EVA) provides relatively high shrink and limited tensile strength/puncture resistance. In some instances blends of various polyethylenes and EVA have been used to obtain a balance of these desirable properties, but with limited success. Monolayer films are suitable for poultry packaging. However for fresh red meat and processed meat packaging, a high degree of oxygen and moisture barrier protection is essential, so the common practice has been to use multilayer films including an oxygen barrier as a core layer between at least one inner layer and one outer layer. Oxygen barriers include vinylidene chloride copolymers such as the vinyl chloride form (saran) and the methyl acrylate form (MA saran), hydrolyzed ethylene vinyl acetate (EVOH) and nylon. The innermost layer of such multilayer films is in direct contact with the stored food and is frequently referred to as the sealing layer. This is because the most common method of sealing heat shrinkable food packages of this type is by heating the mouth edges of the self adhering inner layers together under pressure. This is, the opposite inner surfaces of the film enclosure are sealed by heat. So the inner layer must be formed of a self-adhering material which seals (but does not melt) over an elevated temperature range imparted by commercially used equipment such as impulse sealers. The most commonly used material for the inner layer is EVA, either alone or in a blend with other thermoplastic polymers such as polyethylenes. Typically this inner layer is also the thickest layer of the multilayer film. For example, in a 2.5 mil (0.063 mm) thick film the inner layer of a three layer film may comprise about 60% of this total thickness. The outer layer of oxygen-barrier type multilayer films is often referred to as the abuse layer, because it must withstand abrasion from sliding contact with external objects during normal handling, as well as resistance to puncture from contact with external sharp articles. Outer layers are commonly formed of polyethylenes or EVA or blends thereof. Typically the outer layer of a three layer film comprises 20 - 35%, the barrier layer comprises 10 - 25%, and the inner layer comprises 55 - 70% of the total film thickness. It should also be recognised that food articles such as primal and sub-primal fresh meat cuts often have sharp edges or protruding bones, the multilayer film must have good resistance to puncturing from the inside. It has previously been indicated that whereas EVA affords high biaxial heat shrink properties, its strength properties such as tensile strength and puncture resistance are inferior to polyethylenes such as LLDPE, which unfortunately have relatively low biaxial heat shrink. These limitations have been partially overcome by the discovery of ethylene alpha olefin copolymers known as very low density polyethylene (hereinafter VLDPE), also called "ultra low density polyethylene" (ULDPE) and use of same in heat shrinkable films as for example described in Lustig et al U.S. Patent No. 4,863,769. According to the latter, VLDPE may be used in either the aforedescribed monolayer films for non-barrier poultry shrink packaging or in the inner and/or outer layers of a multilayer oxygen barrier type film. When used in either manner, it provides substantially higher shrink than other polyethylenes such as LLDPE, yet imparts higher strength and puncture resistance than EVA. Accordingly, use of prior art types of VLDPE has permitted the practitioner to achieve biaxial heat shrink properties approaching those of EVA without sacrificing the high tensile strength/puncture resistance properties of polyethylene. In addition to the prior art types of VLDPE, other ethylene alpha olefins are manufactured with elastomeric properties and these commercially available materials are hereinafter referred to as "prior art ethylene alpha olefin plastomers". Some of these have densities in the same range as VLDPE's but have greatly different physical properties due to differences in manufacturing processes. For example, it appears that prior art ethylene alpha olefin plastomers and VLDPE are manufactured with different catalyst systems. One manufacturer of these prior art materials is Mitsui Petrochemicals Ltd., and they sell prior art ethylene-butene plastomer copolymers under the name "Tafmer". These materials are used in blends with other thermoplastic polymers in various applications such as films to provide improved shrink, flexibility, and in some instances improved strength. However, unlike VLDPE's, these prior art ethylene alpha olefins are too weak for use as monolayer biaxially oriented heat shrinkable films in the pure component form. By way of illustration, the prior art Tafmer materials either cannot be extruded into a primary tube, or if this is accomplished cannot be expanded for biaxial orientation by the well-known double bubble method without bubble breakage. While the prior art has improved the heat shrink and physical strength properties of biaxially stretched films (both monolayer and multilayer types) by using blends of thermoplastic polymers such as EVA, prior art VLDPE and prior art ethylene alpha olefin plastomers, there is a need for biaxially stretched films with even higher heat shrink and higher physical strength properties than heretofore available. Moreover in some instances such blending improves physical properties but adversely affects optical properties because of incompatibilities between blended resins. One object of this invention is to provide a heat shrinkable biaxially stretched thermoplastic film with higher shrink properties than obtainable with EVA. Another object is to provide a heat shrinkable biaxially stretched thermoplastic film with higher tensile strength properties than obtainable with prior art polyethylenes. A further object is to provide a heat shrinkable biaxially stretched thermoplastic film with higher dynamic puncture resistance properties than obtainable with prior art polyethylenes. A still further object is to provide a heat shrinkable biaxially stretched thermoplastic monolayer film formed of a single ethylene alpha olefin copolymer, with higher heat shrink, tensile strength and dynamic puncture resistance properties than obtainable with prior art EVA or polyethylenes. Still another object is to provide a heat shrinkable biaxially stretched thermoplastic multilayer film of the oxygen barrier core layer type, with at least one inner or outer layer formed of an EVA-ethylene alpha olefin copolymer blend, with higher shrink, tensile strength and dynamic puncture resistance properties than obtainable with prior art EVA-polyethylene blends, yet good optical properties. Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims. SUMMARY OF THE INVENTION In accordance with the present invention there is provided a heat shrinkable, puncture resistant, biaxially stretched thermoplastic film suitable for use in fabricating bags for packaging food articles. This film comprises a polyethylene which is a linear copolymer of ethylene and higher alpha olefin containing from 4 to 8 carbon atoms, having a density between 0.88 and 0.905 g/cm 3 , a melt index not exceeding 1.5 g/10 min, a molecular weight size distribution ratio Mw/Mn below 3, a melting point below 102°C, a Young's Modulus of below 850 kg/cm 2 , and wherein the film shrinks at least 45 percent at a temperature of about 90°C in at least one of the machine and transverse directions. As used herein, the ethylene alpha olefin copolymers include terpolymers of ethylene and two higher alpha olefins containing 4 to 8 carbon atoms. This film may be a monolayer wherein the above-described ethylene alpha olefin copolymer comprises the entire composition or it may be blended with other thermoplastic polymers. The film may also be part of a multilayer construction such as the previously described oxygen barrier core layer type, wherein the above described ethylene alpha olefin copolymer comprises at least a portion of at least one layer on one side of the core layer. As will be explained hereinafter in more detail and substantiated by the working examples, this film accomplishes all of the aforedescribed objects, and in fact provides additional advantages over prior art films commercially used for shrink packaging food. DETAILED DESCRIPTION OF THE INVENTION The ethylene alpha olefin resins used to manufacture the films of this invention may be prepared by using the traditional Ziegler-Natta heterogeneous catalyst system as for example broadly described in the aforementioned Lustig et al U.S. Patent No. 4,863,769. It is believed that one resin which is useful in this invention, Mitsui's Tafmer A-0585, was prepared by such a catalyst system. Alternatively, the same general type of ethylene alpha olefin resins may be prepared by a homogeneous metallocene single-site catalyst system which in general produces molecular chains of more uniform length with more evenly spaced comonomer. Another result is narrower molecular weight distribution and composition. If necessary, according to the resin manufacturers a multistep polymerisation process with a succession of different metallocene single-site catalysts is capable of producing virtually any desired final molecular weight distribution. It is believed that Exxon Chemical Company's EXACT® Type 3033 (formerly SLP 0633), another ethylene alpha olefin useful in the practice of this invention, was prepared in a metallocene single site catalyst system. This general type of system is described in Welborn U.S. Patent No. 5,183,867 (assigned to Exxon) and European Patent application publication 0 416 815 A2 to G. F. Schmidt (assigned to Dow chemical Company), to which reference is made. Although the ethylene alpha olefin copolymers useful in the practice of this invention are different from previously known VLDPE copolymers, they have at least some of the VLDPE properties as broadly described in the previously referenced Lustig et al U.S. Patent No. 4,863,769 (hereinafter "Lustig et al '769"). Whereas according to Lustig et al '769 the VLDPE may have a density up to 0.914, the ethylene alpha olefins of this invention have densities up to 0.905 g/cm 3 . Likewise, according to Lustig et al '769, VLDPE materials may have densities as low as 0.86 g/cm 3 whereas the ethylene alpha olefins of this invention have densities as low as 0.88 g/cm 3 . Like previously known VLDPE materials, the ethylene alpha olefins useful in this invention may be extruded into monolayer heat shrinkable biaxially oriented films having high strength and high heat shrink. According to Lustig et al '769, this VLDPE shrink is at least 30% at 90°C in at least either the transverse or the machine direction. The films of this invention shrink at least 45% in at least one of the machine and transverse directions at 90°C. Moreover, as will be demonstrated, films of this invention also provide higher puncture strength than films prepared from previously known VLDPE materials. As previously explained, films formed from the ethylene alpha olefin copolymers according to this invention have certain properties which are quite different from and nonanalogous to films containing prior art ethylene alpha olefin plastomers even though one manufacturer of both type of resins, Mitsui, describes them in terms of the same tradename - Tafmer. Table A is a listing of representative prior art ethylene alpha olefin plastomers and their physical properties in the resin form. Ethylene alpha olefin copolymers suitable for use in this invention are relatively high molecular weight, as reflected in the requirement that their melt index not exceed 1.5 g/10 min, as measured by ASTM D-1238, condition E. Prior Art Ethylene Alpha-Olefin Plastomer Physical Properties Type Manufacturer Property Units Value A-4085 Mitsui Melt Index g/10 min 3.6 (D-1238) (190°C) Density g/cc 0.88 (D-1505) Vicat Soften Pt. °C 54 (D-1525) Tensile Strength kg/cm 2 270 At Break (515 K 6301) Elongation % 800 At Break (515 K 6301) Mw g/mole 108,000 Mn g/mole 46,000 Mw/Mn none 2.35 A-4090 Mitsui Melt Index g/10 min 3.6 (D-1238) (190°C) Density g/cc 0.89 (D-1505) Vicat Soften Pt. °C 60 (D-1525) Tensile Strength kg/cm 2 320 At Break (515 K 6301) Elongation % 700 At Break (515 L 6301) Mw/Mn none 2.0 A-1085 Mitsui Melt Index g/10 min 1.4 (D-1238) (190°C) Density g/cc 0.885 (D-1505) Vicat Soften Pt. °C 58 (D-1525) Tensile Strength kg/cm 2 340 At Break (515 K 6301) Elongation % 710 At Break (515 K 6301) Mw g/mole 160,000 Mn g/mole 80,000 Mw/Mn none 2.0 Prior art VLDPE materials include those manufactured by Dow Chemical Company and Union Carbide Corporation, and Table B is a representative list along with physical properties in the resin form. Prior Art VLDPE Physical Properties Type Manufacturer Property Units Value 4001 also XU61520.01 Dow (ethylene-octene-1 copolymer Melt Index D-1238 g/10 min 1.0 Density D-792 g/cc 0.912 Tensile Yield D-638 psi (MPa) 1200 (8.3) Ultimate Tensile D-638 psi (MPa) 3500 (24.1) Ult. Elongation D-638 % 850 Vicat Soften Pt. D-1525 °C 95 Mw/Mn ASTM D-3593 none 5.1 (110,600/21,680) 4003 Dow (ethylene-octene-1 copolymer) Melt Index D-1238 g/10 min 0.8 Density D-792 g/cc 0.905 Tensile Yield D-638 psi (MPa) 950 (6.6) Ultimate Tensile D-638 psi (MPa) 3200 (22.1) Ult. Elongation D-638 % 800 Vicat Soften. Pt. D-1525 °C 80 1137 Union Carbide (ethylene-butene copolymer) Melt Index D-1238 g/10 min 1.0 Density D-792 g/cc 0.905 Tensile Yield D-638 psi (MPa) 2800 (19.3) Ultimate Tensile D-638 psi (MPa) - Ult. Elongation D-638 % 1720 Vicat Soften. Pt. D-1525 °C 80 Mw/Mn ASTM D-3593 none 4.9 125,000/25,700 1192 Union Carbide (ethylene-butene-hexene terpolymer) Melt Index D-1238 g/10 min 0.19 Density D-792 g/cc 0.912 Tensile Strength D-882 psi (MPa) 7100 (49.0) MD 5000 (34.5) TD Ult. Elongation D-882 % 400 MD 760 TD Vicat Soften. Pt. D-1525 °C low 80's reported by mfg. Mw/Mn ASTM D-3593 None 12.2 196,900/16,080 Suitable ethylene alpha olefins for preparing the film of this invention include Mitsui's Tafmer A-0585 and Exxon's EXACT® 3033. The latter material was formerly designated by the manufacturer as SLP 0633, and is an ethylene-butene-hexene terpolymer. Physical properties of Tafmer A-0585, an ethylene-butene copolymer, and EXACT 3033 are set forth in Table C. Tafmer A-0585 Physical Properties Property Unit Value Melt Index (D-1238) g/10 min (190°C) 0.5 Density (D-1505) g/cc 0.88 Melting Point °C 67 Vicat Soften Pt. (D-1525) °C 62 Tensile Strength At Break (515 K 6301) kg/cm 2 400 Elongation At Break (515K 6301) % 650 Mw g/mole 190,000 Mn g/mole 92,600 Mw/Mn none about 2 Exact 3033 Physical Properties Property Unit Value Melt Index (D-1238) g/10 min (190°C) 1.2 Density (D-1505) g/cc 0.900 Melting Point °C 94 Vicat Soften Pt. (D-1525) °C 75-83°C Tensile Strength At Break (D-882) kg/cm 2 9800 MD 9020 TD Mw g/mole 92,000 Mn g/mole 50,000 Mw/Mn none 1.8 Molecular Weight/Size Distribution Most commercial polymeric materials are polydisperse. This means they have a distribution of molecules with varying chain length, which distribution may be characterized by numerical averages. Ethylene alpha-olefin copolymers may be partially characterized by their weight average molecular weight (Mw) which is determined by multiplying the weight of each chain length of a given number of repeat units by the number of such chain lengths present, and dividing by the total weight of chains. Mw is more sensitive to the longer chain length, i.e., larger molecules. Ethylene alpha-olefin copolymers may also be partially characterized by a number average molecular weight (Mn), which is derived from the total number of polymer molecules of each chain length present divided by the total number of chains present. Mn is particularly sensitive to shorter chain lengths, i.e., smaller molecules. When both Mw and Mn are known, they can be used to characterize the shape of the molecular weight distribution curve for the copolymer, i.e. the number of polymer chains in a molecular weight interval as the ordinate and the molecular weight as the abscissa. Qualitatively, a high Mw/Mn connotes a wide distribution of molecular weights whereas a low Mw/Mn connotes narrow distribution. Mw/Mn can be measured by several different techniques, but as used herein the gel permeation chromatography ("GPC") procedure outlined in ASTM D-3593-80 is used. In general, relatively narrow Mw/Mn distribution (and low ratio) permits sharper control of physical properties and superior optical properties, i.e. relatively high gloss and low haze. However, relatively low Mw/Mn ratio value prior art polymers are also difficult to process. More particularly, the specific prior art ethylene alpha-olefin plastomers known to applicant are characterized by relatively narrow molecular weight distribution and Mw/Mn values below about 3. They are relatively difficult to process into films by melt extrusion. Applicant has been unable to prepare biaxially oriented heat-shrinkable film from these materials such as the aforedescribed Tafmer A-1085 and Tafmer A-4085. In contrast, although the ethylene alpha olefins useful in the practice of this invention have relatively low Mw/Mn values below 3, yet they can be used to form pure component biaxially stretched, heat shrinkable film. In general, previously known VLDPE copolymers available in commercial quantities have substantially wider molecular weight distribution and higher Mw/Mn values (i.e. above about 3) than the prior art ethylene alpha olefin plastomers. As such, the previously known VLDPE copolymers have had higher impact strength, tensile strength, abrasion resistance and better processing characteristics. For example, by themselves these VLDPEs may be melt extruded into a primary tube and oriented into a heat shrinkable tube. The molecular weight/size distribution absolute values and Mw/Mn ratios for representative commercially available ethylene alpha-olefin copolymers are set forth in the following Table D. Unless indicated otherwise, these values were measured by the GPC procedure of ASTM D-3593-80. Molecular Weight/Size Distribution Type Compound and Manufacturer Mw/Mn Union Carbide 1085 VLDPE (0.885 density, 0.8 MI) 3.6 (118,000/33,000) Union Carbide 1137 VLDPE (0.900 density, 1.0 MI) 4.9 (125,000/25,700) Union Carbide 1192 VLDPE (0.912 density, 0.19 MI) 12.2 (196,900/16,080) Union Carbide 1096-2 VLDPE (0.912 density, 0.38 MI) 7.2 (137,000/19,110) Dow 4001 VLDPE (0.912 density, 1.0 MI) 5.1 (110,600/21,680) Mitsui 0.896 density VLDPE 4.0 Mitsui 0.907 density VLDPE 3.2 Mitsui Tafmer A-4090 Plastomer 2.0 Mitsui Tafmer A-4085 Plastomer 2.35 (108,000/46,000) Mitsui Tafmer A-1085 Plastomer 2.00 (160,000/80,000) Matsui Tafmer A-0585 Ethylene Alpha Olefin 2.05 (190,000/92,600) Exxon EXACT 3033 Ethylene Alpha Olefin 1.84(92,000/50,000) Melt Index Melt Index (MI) is a measure of the rate of extrusion of molten resins through a die, and reflects the molecular weight as well as the melt strength of the material. That is, relatively low MI values indicate higher molecular weight-higher melt strength resins. For purposes of this invention, suitable ethylene alpha olefin copolymers for use in forming the invention films have melt index values not exceeding 1.5 as determined by ASTM D-1238, condition E (2.16 kg weight) at 190°C. As will be apparent from the following Table E (Melt Indices), this means that from the melt index standpoint these suitable ethylene alpha olefins (Exxon Exact 3033 and Mitsui Tafmer A-0585) are more similar to previously known VLDPE materials than prior art plastomers. Melt Indices Type Compound and Manufacture MI (g/10 min per ASTM D-1238 Condition E) Union Carbide 1085 VLDPE 0.8 Union Carbide 1137 VLDPE 1.0 Union Carbide 1192 VLDPE 0.19 Union Carbide 1096-2 VLDPE 0.38 Dow 4001 VLDPE 1.0 Dow XU61520.01 VLDPE 1.0 Mitsui Tafmer A-4085 Plastomer 3.6 Mitsui Tafmer A-4090 Plastomer 3.6 Mitsui Tafmer A-1085 Plastomer 1.4 Exxon Exact 3027 3.5 Exxon Exact 3033 1.2 Mitsui Tafmer A-0585 0.5 Melting Points There is a substantial difference in melting points between previously known VLDPE and prior art ethylene alpha olefin plastomers. In general, the latter have melting points below 90°C whereas the prior art VLDPE materials have melting points above 90°C. The aforementioned EVA (10% vinyl acetate content) has a melting point of 97°C. One ethylene alpha olefin copolymer suitable for use in this invention (Tafmer A-0585) has a relatively low melting point of about 67°C. Another suitable ethylene alpha olefin copolymer (Exact 3033) has a higher melting point of about 94°C. The melting points of various ethylene alpha olefins and EVA are summarized in Table F. Melting Points (°C) Type Compound and Manufacturer MP (°C) Union Carbide 6833 EVA 97 Union Carbide 1137 VLDPE 117 Union Carbide 1085 VLDPE 117 Union Carbide 1063 VLDPE 124 Union Carbide 1064 VLDPE 125 Union Carbide 1092 VLDPE 121 Dow Attane 4001 VLDPE 121 Dow Attane 4003 VLDPE 107/124 (two peaks) Mitsui Tafmer A-4085 Plastomer 71 Mitsui Tafmer A-4090 Plastomer 85 Mitsui Tafmer A-1085 Plastomer 71 Mitsui Tafmer A-0585 Ethylene Alpha Olefin 67 Exxon Exact 3033 Ethylene Alpha Olefin 94 Ethylene alpha olefins suitable for use as the primary thermoplastic polymer in the film of this invention have a melting point below 102°C. Higher melting polymers do not provide the ultra high shrink characteristics of the inventive film. Vicat Softening Point With the exception of Union Carbide's type 1085 material, previously known as VLDPE materials tend to be less flexible and harder to penetrate than prior art plastomer-type ethylene alpha olefins, which tend to be more flexible and softer. In this regard at least one ethylene alpha olefin useful in the practice of this invention (Mitsui's Tafmer A-0585) is more similar to most prior art ethylene alpha olefin plastomers than previously known VLDPE materials. However, another preferred ethylene alpha olefin (Exxon's Exact 3033) is more similar to the latter than the former. This is illustrated by comparing Vicat softening points for the various materials as measured in accordance with ASTM 1525. By this procedure, Vicat softening point is the temperature at which a flat-ended needle of 1-mm 2 circular cross section will penetrate a thermoplastic specimen to a depth of 1 mm under a specified load using a selected uniform rate of temperature rise. Vicat softening points for various ethylene alpha olefin plastomers have been reported by the resin manufacturers and are summarized in Table G as follows. Vicat Softening Point Type Compound and Manufacturer VSP (°C) per ASTM-1525 Union Carbide 1085 VLDPE 45 Union Carbide 1137 VLDPE 80 Union Carbide 1063 VLDPE 93 Union Carbide 1064 VLDPE 90 Union Carbide 1569 VLDPE 94 Dow Attane 4001 VLDPE 95 Dow Attane 4003 VLDPE 80 Dow Attane 4004 VLDPE 92 Mitsui Tafmer A-1085 Plastomer 58 Mitsui Tafmer A-4085 Plastomer 54 Mitsui Tafmer A-4090 Plastomer 65 Mitsui Tafmer A-20090 Plastomer 60 Mitsui Tafmer A-0585 Ethylene Alpha Olefin 62 Exxon Exact 3033 Ethylene Alpha Olefin 75-83 Tensile Properties One of the characteristics of ethylene alpha olefins useful in the practice of this invention is the ability to absorb tensile stresses without rupturing. In particular the density should be between 0.88 and 0.905 g/cm 3 , so that stresses are absorbed in the more numerous short chain branches present in the relatively high alpha olefin comonomer content of the copolymer. On the other hand lower density resins are too rubbery for processing into a biaxially stretched film. Another important property for characterizing suitable ethylene alpha olefin copolymers is its Young's Modulus, sometimes referred to as the modulus of elasticity, and associated with a polymer undergoing tensile deformation. It is the ratio of stress to strain below the proportional limit of a material. Stated otherwise, it is the stiffness or a measure of the resistance to tensile deformation that a polymer exhibits when a tensile stress is applied. Stiff materials have high values whereas low to moderate values indicate more elasticity and flexibility. For purposes of this invention, suitable ethylene alpha olefins have a Young's Modulus below 850 kg/cm 2 . These materials, like the prior art ethylene alpha olefin plastomers, can absorb more of the energy imparted by a projectile, leading to higher values of dynamic puncture. Also their elasticity allows them to be biaxially stretched to a higher extent, leading to higher shrink in films. Mitsui, a manufacturer of ethylene alpha olefin plastomers, Tafmer A-0585 and previously known VLDPE materials, reports the following values for Young's Modulus as summarized in Table H. The latter also includes Young's Modulus for Exxon's Exact 3033. Young's Modulus Material Kg/cm 2 Tafmer A-0585 400 Tafmer A-4085 400 Tafmer A-20090 600 VLDPE (0.896 density) 800 VLDPE (0.907 density) 1820 Exact 3033 773 From Table H, one would expect Tafmer A-0585 (like the prior art Tafmer-type ethylene alpha olefin plastomers), to be too weak to form monolayer biaxially stretched heat shrinkable films therefrom. It was in fact quite surprising that this could be done with Tafmer A-0585 to produce such film with commercially acceptable tensile strength properties and very high puncture resistance. It appears that Exact 3033 has a substantially higher Youngs Modulus value that Tafmer A-0585. Shrinkage The biaxially stretched film of the present invention are "heat-shrinkable", and as used herein this means the film has an unrestrained shrinkage of at least 45 percent, e.g. at least 50 percent or at least 55 percent, in at least one of the machine and transverse directions at 90°C. Shrinkage values are obtained by measuring unrestrained shrink of the stretch film at 90°C for five seconds. Four test specimens are cut from a given sample of the stretched film to be tested. The specimens are cut to 10 cm in the machine direction by 10 cm in the transverse direction. Each specimen is completely immersed for 5 seconds in a 90°C water bath. The distance between the ends of the shrunken specimen is measured. The difference in the measured distance for the shrunken specimen and the original 10 cm is multiplied by ten to obtain the percentage of shrinkage for the specimen. The shrinkage for the four specimens is averaged for the MD shrinkage values of the given film sample, and the shrinkage of the four specimens averaged for the TD shrinkage value. The films of this invention are generally prepared from dry resins which are melted in an extruder and passed through a die to form the primary film material, most commonly in tube form. The well-known two step "double bubble" or trapped bubble process may be used to prepare the film of this invention (either monolayer or multilayer). One embodiment of this invention is described in Pahlke U.S. Patent No. 3,456,044. In this Pahlke-type double bubble process a primary tube is prepared, cooled, reheated and the tube is simultaneously stretched in the machine direction ("MD") by operating longitudinally spaced nip rolls at different speeds, and in the transverse direction ("TD") by inflating air inside the tube. Suitable stretch ratios are from 2 to 6 with ratios of 3 to 5 preferred. The multilayer heat-shrinkable, puncture resistant biaxially stretched thermoplastic films of this invention may be formed into a primary tube by known techniques such as by coextruding at least the core layer and the first and second outer layers on each side of the core layer to form a primary tube as for example described in Canadian Patent No. 982923. Alternatively, the composite primary tube may be formed by coating lamination, wherein a first outer tubular layer is extruded and thereafter the core and second outer tubular layers are sequentially coated onto the outer surfaces of the first tubular layer and the core layer. As another alternative, the first outer and core outer layers may themselves be coextruded, and the second outer layer thereafter coated onto the outside surface of the core layer. Coating lamination procedures are described in Brax et al U.S. Patent No. 3,741,253. As still another alternative, the at least three layer film may be formed as a sheet by the well-known slot casting procedure. The preferred method for forming the films of this invention (both monolayer and multilayer) is coextrusion of a primary tube which is then biaxially oriented in the manner described in the aforementioned Pahlke Patent. In the following multilayer film Examples 2-5, all three layers were coextruded and the primary tube was cooled. It was then reheated to the draw temperature for biaxial orientation. Although not essential in the practice of this invention, it may be desirable to cross-link a monolayer film embodiment or one or more layers of a multilayer embodiment of the inventive film for improvement of abuse and/or puncture resistance and other physical characteristics. This for example may be accomplished by irradiation using high energy electrons, ultra violet radiation, x-rays, beta particles and the like. The irradiation source may be any electron beam generator operating in a range of 150 kilovolts to 6 megavolts with a power output capable of supplying the desired dosage. The voltage can be adjusted to appropriate levels which may be for example 1 to 6 million volts or higher or lower. Many apparatus for irradiating films are known to those skilled in the art. The irradiation is usually carried out at a dosage of up to 20 MR, typically between 1 MR and 20 MR, with a preferred dosage range of 2 MR to 12 MR. Irradiation can be carried out conveniently at room temperature, although higher and lower temperatures, for example, 0°C. to 60°C. may be employed. Processwise, irradiation can be applied to a single substrate layer such as the first outer layer and prior to biaxial stretching if the primary multilayer film is made by coating lamination. This type of irradiative cross-linking is for example described in the aforementioned Brax et al U.S. Patent U.S. 3,741,253. Alternatively, if the entire film is simultaneously coextruded it may be preferable to irradiate the entire multilayer film, and for maximum efficiency this should be done after biaxial stretching at a dosage below 8 MR, as for example described in Lustig et al U.S. Patent No. 4,737,391. Cross-linking may also be accomplished chemically through utilization of peroxides, as is well know to those skilled in the art. A general discussion of cross-linking can be found at pages 331 to 414 of volume 4 of the Encyclopedia of Polymer Science and Technology, Plastics, Resins, Rubbers, Fibres published by John Wiley & Sons, Inc. and copyrighted in 1966. The document has a Library of Congress Catalogue Card Number of 64-22188. Unless otherwise noted, the resins utilized in the present invention are generally commercially available in pellet form, and as generally recognized in the art, may be melt blended or mechanically mixed by well known methods using commercially available equipment including tumblers, mixers and blenders. Also, if desired, well known additives such as processing aids, slip agents, antiblocking agents and pigments and mixtures thereof may be incorporated into the film, by blending prior to extrusion. The resins and any additives are introduced to an extruder (generally one extruder per layer) where the resins are melt plastified by heating and then transferred to an extrusion (or coextrusion) die for formation into a tube. Extruder and die temperatures will generally depend upon the particular resin or resin containing mixtures being processed and suitable temperature ranges for commercially available resins are generally known in the art, or are provided in technical bulletins made available by resin manufacturers. Processing temperatures may vary depending upon other process parameters chosen. In extrusion of monolayer films according to this invention, extruder barrel and die temperature may be as high as the melting point of the ethylene alpha olefin being processed. However depending on such factors as other resins which may be employed, the manufacturing process used and particular equipment and other process parameters utilized, variations are expected. Actual process parameters including process temperatures will be set by one skilled in the art without undue experimentation. For example, in the 100% Exact 3033 sample 3A the extruder barrel temperature range was 350 - 400°F (177 - 204°C) and the die temperature was 345 - 355°F (174 - 179°C), whereas in the 100% Tafmer A-0858 sample 3 the extruder barrel temperature was 400 - 450°F (204 - 232°C) and the die temperature was about 400°F (204°C). Advantageously a preferred embodiment of the inventive heat shrinkable film will have a maximum extractable portion of 5.5 percent by weight of polymer in n-hexane at 50°C for 2 hours as further described below. This 5.5 weight percent represents the desired maximum n-hexane extractable limit for olefin copolymers of the type employed by the present invention for use in articles that contact food except for articles used for packing or holding food during cooking. Beneficially the maximum extractable portion as described above will be 2.6 percent in an especially preferred embodiment of the inventive film suitable for use in articles used for packing or holding food during cooking. The above maximum extractable values correspond to current limits for a class of resins intended for use in contact with food as set forth and described by the U.S. Food & Drug Administration in 21 CFR 177.1520 to which reference is made. Mitsui, the manufacturer of Tafmer products, advises that these products meet these extractable limits (in the film surface contacting food) if Tafmer comprises no more than 40% of a blend with EVA or certain polyethylenes. In all of the following examples, all films were prepared by the double or trapped bubble method as broadly described in the aforementioned Pahlke U.S. Patent No. 3,456,044. In the multilayer embodiments of examples 2 - 5, all three layers were simultaneously coextruded, cooled and then reheated for biaxial orientation. The VLDPE used was the aforedescribed Attane XU 61520.01 or 4001, both manufactured by the Dow Chemical Company of Midland, Michigan, and comprising an ethylene and 1-octene copolymer with a density of about 0.912 gm/cc. The EVA was Union Carbide Corporation's type 6833 with 10% vinyl acetate content. The extrusion-biaxial stretching equipment used in Examples 1 - 3 was laboratory (small) scale whereas the extrusion equipment used in Examples 4 and 5 was commercial size (large size). EXAMPLE 1 In Example 1, seven different pure component resins were used in attempts to manufacture heat-shrinkable biaxially stretched monolayer films in tube form, and four of the runs were successful. Five of these resins were included for comparison with film of this invention, i.e. sample 1 (Tafmer A-4085), sample 2 (Tafmer A-1085), sample 4 (Exxon's Exact 3027), sample 5 (EVA) and sample 6 (Dow's type 61520.01 VLDPE). Sample 3A (Tafmer A-0585) and sample 3B (Exact 3033) are ethylene alpha olefins suitable for manufacturing film of this invention. Tafmer types A-4085 and A-1085 have already been described as prior art ethylene alpha olefin plastomers. Exact 3027 is an ethylene butene copolymer manufactured by Exxon Chemical company using a homogeneous metallocene single-site catalyst system. It is reported by the manufacturer as having a melt index of 3.5 (2 kg at 190°C), density of 0.900 gm/cc, a single melting point of about 92°C, and an Mw/Mn of about 2. Exxon classifies its Exact materials with densities of at least 0.900 gm/cc as VLDPE. The films were all prepared in the same extrusion-orientation system of the general type disclosed in the aforementioned Pahlke Patent. The primary tube was about 2 inches (6.35 cm) flat width inside diameter, and final heating to the draw temperature was by an air cushion which had itself been heated by transverse flow through a heated porous tube concentrically positioned around the moving primary tube. The final tube was about 12 inches (30 cm) inside diameter and the film thicknesses were about 2 mils (0.05 mm). Physical properties of the resulting monolayer films were measured and are summarized in Table I. Referring now to Table I, sample 1 (100% Tafmer A-4085), sample (100% Tafmer A-1085) and sample 4 (100% Exact 3027) could not be formed into film tubes, yet sample 3A (100% Tafmer A-0585) formed a heat shrinkable biaxially stretched film with outstanding physical properties. This will be apparent from a comparison with the physical properties of sample 5 (100% EVA - 10% vinyl acetate) and sample 6 (100% VLDPE 0.912 density) heat shrinkable biaxially stretched films. More specifically, even though the physical properties of Tafmer A-4085, Tafmer A-1085 and Tafmer A-0585 resins are very similar, it was unexpectedly discovered that a monolayer heat shrinkable biaxially stretched film with uniquely advantageous physical properties (relative to other ethylene alpha olefin copolymers) only could be prepared from Tafmer A-0585. For example, its tensile strength and elongation characteristics are at commercially acceptable levels, yet its shrink properties far surpass those of 100% EVA. This was very surprising because EVA has been generally recognised as providing the highest biaxial shrink properties of widely used ethylene copolymers. Moreover the dynamic puncture resistance of 100% Tafmer A-0585 sample 3A was unexpectedly several times higher than 100% VLDPE sample 6. This was unexpected from a comparison of their resin physical properties, and particularly since prior art polyethylene films of this general type have been recognised by those skilled in the art as providing the highest strength properties of ethylene alpha olefin-based heat shrinkable biaxially stretched films. Sample 3A demonstrates a preferred monolayer embodiment having puncture strength of at least 10 cmkg/mil (2520 cmkg/cm). It should be noted, however, that because of its relatively low melting point (67°C), 100% Tafmer A-0585 film is not suitable for systems wherein the film is heat shrunk by contact with boiling water. Also its optical properties are somewhat inferior to the prior art film samples 5 and 6. It is believed that the reasons why heat shrinkable biaxially oriented monolayer film can be prepared from Tafmer A-0585 but not from Tafmer A-4085 or Tafmer A-1085 or Exact 3027 are the aforedescribed differences in melt index and possibly Mw and Mn. Compared to prior art VLDPE, the remarkably higher shrink and dynamic puncture values are believed due to differences in density, melting point, Mw/Mn, and possibly Mw and Mn. Finally compared to EVA sample film 5, the much higher shrink performance of invention film sample 3 is believed in part due to its greater elasticity. Sample 3B (100% Exact 3033) is another ethylene copolymer which has unexpectedly superior properties compared to prior art elastomeric ethylene alpha olefins such as Tafmer A-4085 Sample 1. That is, it formed a heat shrinkable biaxially stretched film with certain outstanding physical properties. For example, although its tensile strength and elongation characteristics were somewhat inferior to the other monolayer samples 3A, 5 and 6, its shrink properties were comparable to 100% EVA (sample 5) and substantially better than 100% prior art VLDPE (sample 6). Moreover its dynamic puncture resistance of 38 cmkg/mil (14961 cmkg/cm) was far superior to the other films and over three times higher than the 100% Tafmer 0585 film sample 3A embodiment of the invention. The latter in turn has about four times the puncture resistance of the best performing prior art film (VLDPE sample 6). The optical properties of Exact 3033 film sample 3B are comparable with the best prior art film in this regard (EVA sample 5). Table I demonstrates that monolayer films of this invention are unexpectedly superior to prior art films used in shrink packaging food products wherein an oxygen barrier is not required, eg. poultry such as chicken. Accordingly, with acceptably low n-hexane extractables these films would also be suitable for such end uses. Example 2 In Example 2, five nonirradiated multilayer films (samples 7 - 10) were prepared each with a vinylidene chloride copolymer type core layer sandwiched between two identical outer layers. All films were the heat shrinkable biaxially stretched type of about 2.0 mils (0.051 mm) overall thickness. One outside layer (the inner layer of the tube which would become the inner and heat sealing layer of bags formed from the tube) was about 1.2 mils (0.03 mm) thick. The other outside layer (the outer layer of the tube which would become the outer abuse-resistant layer of bags formed from the tube was about 0.6 mils (0.015 mm) thick. The purpose of Example 2 was to compare physical and optical properties of three layer films of the oxygen barrier core layer type prepared in a small scale extrusion biaxial stretching system. In these films the outer layers were formed of blends including ethylene alpha olefin compositions according to this invention (samples 8 - 10), and a prior commercially employed VLDPE-EVA blend (sample 7) was used as the control. The films were all prepared in the same small scale orientation system of the general type described in the Pahlke Patent. The extruder barrel and die temperatures were 320 - 330°C. The primary tube was about 4 1/8 inch (10.3 cm)inside diameter and the final biaxially stretched tube was about 19 inches (47.5 cm)inside diameter. Physical and optical properties of the resulting nonirradiated three layer films were measured and are summarized in Table J. Summarizing Table J, samples 8A, 8B, 9 and 10 are embodiments of the invention and may be compared with sample 7, a heat shrinkable biaxially stretched film used after irradiation and in bag form for packaging fresh red meat and processed meats. Since they contain about 70% type 4001 VLDPE, sample 8A (69.1% Tafmer A-0585) and sample 8B (70% Exact 3033) may be directly compared with control sample 7 because in each instance the balance of the composition is primarily EVA plus a small percentage of processing aids. Both invention samples 8A and 8B have substantially higher MD tensile strength and shrink in both directions than prior art sample 7. Moreover, the dynamic puncture resistances of samples 8A and 8B are severalfold higher than prior art VLDPE sample 7. This was surprising because, as previously stated, prior art VLDPE is known to provide the highest puncture resistance to the ethylene alpha olefin copolymer type inner and outer layers of state-of-art multilayer oxygen barrier type films. The optical properties of the 70% Exact 3033 sample 8B were far superior to the 70% VLDPE control sample 7, whereas the hot water puncture value of sample 8A was relatively low because of the low Tafmer A-0585 melting point. However, far superior results were obtained by blending Exact 3033 and Tafmer A-0585 (discussed below). The optical properties of the 70% Exact 3033 sample 8B were far superior to the 70% VLDPE control sample 7, whereas the 69% Tafmer A-0585 film sample 8A optical properties were slightly better than prior art sample 7. These improvements are believed to be related to differences in density, Mw/Mn, possibly Mw and Mn, melting point and Young's Modulus. Samples 9 (40% Tafmer A-0585, 33% Exact 0633, and 23% EVA) and 10 (55% Exact 0633 and 40% Tafmer A-0585) demonstrate that physical properties may be adjusted by blending the ethylene alpha olefin copolymer of this invention either in a two component blend or in a three component blend with at least one other constituent such as EVA. More particularly, sample 9 provides higher shrink force, hot water puncture and superior opticals relative to 69% Tafmer A-0585 sample 8A, by replacing some of the Tafmer A-0585 with Exact 3033. However, the tensile strength, shrink and dynamic puncture values are made somewhat lower by the substitution. Sample 10 demonstrates that a two component blend provides the best balance of all Table I invention embodiments. Accordingly, one preferred embodiment of this invention involves use of a blend comprising the aforedescribed Tafmer A-0585 ethylene alpha olefin copolymer, and another ethylene alpha olefin copolymer prepared by a metallocene single site catalyst-Exact 3033. Table J shows that with respect to nonirradiated multilayer films prepared in small scale equipment, the three layer-oxygen barrier core type with the present ethylene alpha olefin copolymers in the inner and outer layers are superior in terms of certain physical properties to comparable prior art films currently used for shrink packaging fresh red meat and processed meat. Since the other physical properties of these inventive films are commercially acceptable, they are also suitable for these end uses. This of course presumes that if the film is intended for direct contact with food it will satisfy the n-hexane extractable limit. As will be discussed below in more detail in connection with Example 3, the physical properties of the nonirradiated Tafmer A-0585 and Exact 0633 blend sample 10 is actually superior to its irradiated counterpart. Accordingly, in some end uses where the evacuated packages are sealed by clipping or where the sample 10 inner layer heat seal temperature range is adequate, irradiation may be avoided. Example 3 In Example 3, five irradiated multilayer films (samples 11 - 14) were prepared each with a vinylidene chloride copolymer-type oxygen barrier core layer sandwiched between two identical outer layers. All films were the heat shrinkable biaxially stretched type of about 2.0 mils (0.051 mm) overall thickness. The heat sealing outer layer was about 1.2 mils (0.030 mm) thick and the abuse-resistant outer layer was about 0.6 mils (0.015 mm) thick. The purpose of Example 3 was to compare physical and optical properties of irradiated three layer films of the oxygen barrier core layer type. In these films the outer layers are formed of blends including ethylene alpha olefin compositions according to this invention (samples 12 - 14) and a prior art commercially employed VLDPE - EVA blend (sample 11) used as the control. The same small scale extrusion - orientation system was used to prepare each of the samples 11 - 14, and it was the same system used in example 2. The biaxially stretched films were irradiated to about 4MR (after orientation) in the manner described in Lustig et al U.S. Patent No, 4,737,391. Physical and optical properties of the resulting irradiated three layer films were measured and are summarized in Table K. Summarizing Table K, samples 13 - 14 are embodiments of the invention and may be compared with sample 11, an irradiated heat shrinkable biaxially stretched film used in bag firm for packaging fresh red meat and processed meats. Since control sample 11 contains about 70% type 4001 VLDPE, sample 14A (Tafmer A-0585) and sample 14B (Exact 3033) may be directly compared because in each instance the remainder is primarily EVA and a small amount of processing aids. Samples 14A and 14B have substantially higher tensile strength (14A only in MD) and shrink in both directions. Moreover the dynamic puncture resistance of both samples 14A and 14B are substantially higher than control sample 11. Finally, the optical properties of both samples 14A and 14B are superior to control sample 11. Samples 14A and 14B illustrate a preferred embodiment of this invention comprising an irradiated biaxially stretched film with three layers. It includes a vinylidene chloride-methyl acrylate copolymer oxygen barrier core layer and an outer layer on each side of the barrier core layer. The outer layers each comprise a blend of between about 65% and 70% of the selected ethylene alpha olefin, and between about 20% and 30% ethylene vinyl acetate. The film preferably has dynamic puncture strength of at least 3 cmkg/mil (1181 cmkg/cm). Improvements over the control sample 11 are also achieved by substituting a different ethylene alpha olefin copolymer (Exact 3033) for a portion of the Tafmer A-0585 ethylene alpha olefin of this invention to form a three component blend (sample 12), or using a two component blend of Exact 3033 and Tafmer A-0585 in the inner and outer layers (sample 13). However, comparison of physical properties with nonirradiated sample 10 (Table J) having the same composition suggests that since the irradiated film properties are inferior, irradiation should only be employed if needed to broaden the heat sealing range of the inner layers and improve the hot water puncture resistance. Since the control prior art film is commercially used to shrink package fresh red meat and processed meat and the invention film samples have certain improved physical properties compared to this prior art film and the remaining physical properties are commercially acceptable, these film embodiments of the invention are suitable for the same end use. Example 4 In Example 4, five nonirradiated biaxially stretched and heat shrinkable multilayer films (samples 15 - 18) were prepared, each with a vinylidene chloride copolymer-type oxygen barrier core layer sandwiched between two identical outer layers. All films were the heat shrinkable biaxially stretched type of about 2 mils (0.051 mm) overall thickness. The heat sealing outer layer was about 1.2 mils (0.03 mm) thick and the abuse-resistant outer layer was about 0.6 mils (0.015 mm) thick. Samples 15 (Dow type XU 61520.01 VLDPE - EVA blend outer layers) 17 (100% Dow type 4001 VLDPE outer layers) and 18 (100% EVA outer layers) are film compositions commercially used in the irradiated form for meat packaging. Samples 16A (91.6% Exact 3033) and sample 16B (Exact 3033 - Tafmer A-0585 blend) are embodiments of this invention. A large scale extrusion-biaxial stretching system of the Pahlke Patent type was used to prepare samples 15-18. The primary tube was about 2 inches (2.5 cm) inside diameter flat width whereas the final biaxially stretched tube flat width was about 10 inches (25.4 cm) inside. Final heating to the draw temperature was by an air cushion which had itself been heated by transverse flow through a heated porous tube concentrically positioned around the moving primary tube. The purpose of Example 4 was to compare physical and optical properties of nonirradiated three layer films of the oxygen barrier core layer type, prepared in a large scale commercial type extrusion-biaxial stretching system. Physical and optical properties of the resulting nonirradiated three layer film were measured and are summarized in Table L. Referring now to Table L, sample 16A (91.6% Exact 3033) and sample 16B (56.0% Exact 0633-40.0% Tafmer A-0585) are invention embodiments whereas samples 15 (69% VLDPE and 22.5% EVA), 17 (100% VLDPE) and 18 (100% EVA) are prior art films. Since invention embodiment sample 16A has 91.6% Exact 3033 in the inner and outer layers it may be compared with the 100% EVA and 100% VLDPE prior art films. The sample 16A overall tensile strength is significantly higher than that of the highest prior art film. Moreover the sample 16A shrink in both directions is significantly higher than the highest heat shrinkable prior art film (EVA sample 18). The dynamic puncture resistance of 91.6% Exact sample 16A is substantially higher than that of the prior art films (VLDPE sample 17). Sample 16B demonstrates that certain improvements in physical properties over comparable prior art film may be realized by using two component blends of the present ethylene alpha olefin copolymer of this invention and another ethylene alpha olefin copolymer of this invention prepared by a metallocene single site catalyst system, in the inner and outer layers. In particular, sample 16B has the highest shrink in both directions as well as the highest dynamic puncture value of all film samples in this test series. Its optical properties were nearly as good as the best prior art film from this standpoint (sample 17). Example 5 In Example 5, seven irradiated multilayer films (samples 19-23) were prepared with a vinylidene chloride copolymer-type oxygen barrier core layer sandwiched between two identical outer layers. The overall thickness of the film was about 2 mils (0.051 mm), the heat sealable outer layer thickness was about 1.2 mils (0.03 mm) and the abuse resistant outer layer was about 0.6 mils (0.015 mm) thick. Sample 19 (Dow XU 61520.01 type VLDPE-EVA blend outer layers) and sample 23A (100% EVA outer layers) are commercially used for meat packaging and are the control films. Samples 20A and 20B, 21, 22 and 23B have outer layers containing either Tafmer A-0585, Exact 3033 or both and are embodiments of this invention. All films were post-irradiated to about 4 MR in the manner generally described in previously referenced Lustig et al patent '391. The same large scale extrusion-orientation system used in Example 4 was used in Example 5. That is, the primary tube inside diameter was about 2 inches (2.5 cm) and the final biaxially stretched tube diameter was about 10 inches (25.4 cm) inside. The purpose of Example 5 was to compare physical and optical properties of invention embodiments with prior art irradiated three layer films of the oxygen barrier core layer type commercially used for shrink packaging of meat, wherein the films were prepared in a large scale commercial type extrusion-biaxial stretching system. Physical and optical properties of the resulting irradiated three layer film were measured and are summarized in Table M. Referring now to Table M, invention film samples 20A and 20B may be compared with control sample 19 because each has about 70% ethylene alpha olefin copolymer. In each instance the balance of the composition is primarily EVA with a small percentage of processing aids. With respect to tensile strength, sample 20B (69.1% Exact 3033) has a very high value. The tensile strength of invention sample 20A (70% Tafmer A-0585) is considerably lower than the Exact 3033 film sample 20B but about the same as the commercially used 100% EVA inner-outer layer film sample 23A. The dynamic puncture strength of Tafmer A-0585 invention sample 20A is substantially higher than prior art VLDPE sample 19 or Exact 3033 invention sample 20B. In terms of optical properties, the 69% Exact 3033 sample 20B of the invention was substantially superior to all of the prior art films whereas the 70% Tafmer A-0585 invention sample 20A was somewhat inferior. With respect to shrink, invention film samples 20A and 20B are far superior to previously known both VLDPE sample film 19, and in fact superior to 100% EVA inner-outer layer film sample 23A. As previously explained, this was surprising because EVA is known in the art as providing the best shrink of commercially used ethylene copolymer inner and outer layer type oxygen barrier multilayer films. Sample 20A demonstrates a preferred embodiment of the invention wherein the film shrinks at least 55% in both the machine and transverse directions at 90°C. Table M also demonstrates that invention film sample 23B (91.6% Exact 3033) is superior to 100% EVA sample 23A as well as the aforementioned prior art VLDPE sample 19 in certain respects. For example, its shrink (in both directions) and dynamic puncture is superior to both of the Table M prior art films. However, its optical properties are somewhat inferior. Invention film sample 21 (55% Exact 3033 and 40% Tafmer A-0585) demonstrates that a blend of two ethylene alpha olefin copolymers of this invention including one prepared by a metallocene single site catalyst system (Exact 3033) in the inner and outer layers provides substantially higher shrink than the best performing comparable prior art film (100% EVA sample 23A). Invention film embodiment sample 22 demonstrates that a three component blend of 40% Tafmer A-0585, 33% Exact 3033 and 23% EVA provides certain improved physical properties. For example, its shrink (in both directions) is substantially higher than the best performing prior art film (100% EVA sample 23). Also, its dynamic puncture strength is substantially higher than the best performing prior art film (69.1% previously known VLDPE sample 19). However, its optical properties are somewhat inferior. Since the control films are commercially used to package fresh red meat and processed meat, and the Table M invention embodiments are superior in terms of certain physical properties and commercially acceptable with respect to the remaining physical properties, the latter are also suitable for the same end use if they satisfy n-hexene extractable limits. In the Example 2-5 comparisons with films prepared from previously known VLDPE, the latter's density was about 0.912 g/cm 3 . Lower density VLDPE materials are also commercially available and provide films with relatively higher shrink values than a higher density VLDPE prepared from the same alpha olefin comonomer, eg. C8. The basic reason for this difference is that lowering density increases film flexibility. However, their shrink and puncture strength properties are inferior to the previously described invention embodiments. Accordingly, previously known lower density VLDPE materials such as those of about 0.910 g/cm 3 and below (and having melt indices of the order of about 1.0 g/10 min and below), cannot be used to prepare films having properties equivalent to the films of this invention. This was demonstrated in a series of three layer film tests wherein the barrier layer was identical to the Examples 2 - 5 films and the outer layers comprised about the same percentages of previously known VLDPE (approximately 70%) and EVA (25% Union Carbide type 6833) as in several samples of Examples 2 - 5. The control film of these tests included the same 0.912 density Dow type 4001, used for example in previously described samples 7 and 15. Two other previously known VLDPE materials were used: Dow's type XU 61512.21 having 0.901 g/cm 3 density and 1.0 g/10 min melt index, and Dow's type XU 61509.32 having 0.911 g/cm 3 density and 0.5 melt index. In each instance the comonomer was octene-1. Example 6 In Example 6, four nonirradiated heat shrinkable multilayer film samples 24 - 27 were prepared by the previously described coextrusion-type double bubble process in laboratory (small) scale equipment. In each instance the primary tube was about 3.25 inches (8.3 cm) flat width inside diameter, the final tube was about 13 inches (33 cm) inside diameter and the film thicknesses were about 2 mils (0.051 mm). Physical and optical properties of these films were measured and the results are summarized in Table N. The latter shows that sample 27 (0.901 density previously known VLDPE) provides substantially higher heat shrink than samples 24 and 25 (0.912 density previously known VLDPE). As previously explained, this was because of the greater flexibility afforded by lower density material. However lower density VLDPE sample 27 also has substantially lower dynamic puncture strength than the highest performing 0.912 density VLDPE sample 24. The shrink and dynamic puncture strength characteristics of 0.911 density VLDPE sample 26 are substantially the same as 0.912 density VLDPE sample 24. The Table N results may be qualitatively compared with the Example 2 (Table J) results since both sets of film samples were prepared in small scale equipment, have the same type formulations with the same 0.912 density VLDPE-containing outer layer control sample, and are nonirradiated. More particularly sample 8A (69.1% Tafmer A-0585, 22.5% EVA) has very high shrink (56/52%) and dynamic puncture strength (8 cmkg/mil) (3150 cmkg/cm) compared to 0.912 density VLDPE control sample 7. Low density VLDPE sample 27 also has substantially lower heat shrink (36/41%) than the present invention (at least 50% in at least one direction). Also the dynamic puncture strength (2.59 cmkg/mil) (1020 cmkg/cm) is only about one third as high as the aforementioned invention sample 8A. Example 7 In Example 7, three nonirradiated heat shrinkable film samples 28 - 30 were prepared by the previously described coextrusion-type double bubble process in a large scale (commercial type) system. The primary tube was about 2 inches (5.1 cm) inside diameter flat width, the final biaxially stretched tube flat width was about 10 inches (25.4 cm) inside and about 2 mils (0.051 mm) thick. Physical and optical properties of these films were measured and the results are summarized in Table O. The latter shows that sample 29 (0.901 density previously known VLDPE) provides higher shrink than sample 28 (0.912 density previously known VLDPE) or sample 30 (0.911 density previously known VLDPE). As previously explained, this was probably due to the greater flexibility of the lower density material. The Table O results may be qualitatively compared with the Example 4 (Table L) results since both sets of three layer film samples were prepared in very similar large scale equipment, have the same type formulation with the same 0.912 density VLDPE-containing outer layer control sample, and are nonirradiated. More particularly, invention film embodiments sample 16A (91.6% Exact 3033) sample 16B (40% Tafmer A-0585, 56% Exact 0633) have much higher puncture strengths than previously known 0.912 density VLDPE-containing sample 15. Accordingly, it is believed that a comparable embodiment of the inventive film would have substantially higher puncture strength than a 0.901 density previously known VLDPE-containing outer layer type film such as sample 30. From the shrink standpoint, invention samples 16A and 16B have very high values compared to sample 15, which is substantially the same composition as control sample 28 of this Example 7. Accordingly, a comparable embodiment of the inventive film would have substantially higher shrink values than the 32/39% values of sample 30. Although preferred embodiments of this invention have been described in detail, it is contemplated that modifications thereof may be made and some preferred features may be employed without others, all within spirit and scope of the broad invention.

Claims: A heat shrinkable puncture resistant, biaxially stretched thermoplastic film suitable for use in fabricating bags for packaging food articles comprising a polyethylene which is a linear copolymer of ethylene and higher alpha olefin containing from 4 to 8 carbon atoms, having a density of between 0.88 and 0.905 g/cm 3 , a melt index not exceeding 1.5 g/l0min, as determined by ASTM D-1238, condition E, a molecular weight/size distribution ratio Mw/Mn below 3, as measured by the GPC procedure of ASTM D-3593-80, a melting point below 102°C, a Young's Modulus of below 850 kg/cm 2 , and wherein the film shrinks at least 45 percent at a temperature of about 90°C in at least one of the machine and transverse directions, as determined by measuring unrestrained shrink of the stretch film at 90°C for 5 seconds. A film according to claim 1 wherein said film is monolayer. A film according to claim 1 wherein said film is multilayer, and the ethylene alpha olefin copolymer comprises at least one layer of said film. A film according to claim 1 wherein said film is multilayer wherein one layer is an oxygen barrier material as core with at least one other layer on each side thereof, and the ethylene alpha olefin copolymer comprises at least one layer of said other layers. A film according to claim 4 wherein the oxygen barrier core layer is a copolymer of vinylidene chloride, such as vinylidene chloride-vinyl chloride copolymer or vinylidene chloride-methyl acrylate, or hydrolysed ethylene vinyl acetate. A film according to any one of claims 1 to 5 wherein the ethylene alpha olefin copolymer is blended with at least one other thermoplastic polymer, such as another ethylene alpha olefin copolymer and/or ethylene vinyl acetate. A film according to any one of claims 1 to 6 wherein the ethylene alpha olefin copolymer and/or said another ethylene alpha olefin copolymer is prepared by a metallocene single site catalyst system. A film according to claim 6 or 7 wherein said another ethylene alpha olefin copolymer is a linear copolymer of ethylene and higher alpha olefin containing from 4 to 8 carbon atoms, has a density of between 0.88 and 0.905 g/cm 3 , a melt index not exceeding 1.5 g/10 min, a molecular weight/size distribution ratio Mw/Mn below 3, a melting point below 100°C, and a Young's Modulus of below 850 kg/cm 2. A film according to any one of claims 1 to 7 wherein said ethylene alpha olefin copolymer has a Vicat softening point not exceeding 70°C, as measured in accordance with ASTM D-1525. A film according to claim 1 wherein ethylene alpha olefin copolymer is a butene copolymer having a density of about 0.88 g/cm 3 , melt index of about 0.5 g/10 min, an Mw value of about 190,000, an Mn value of about 92,600, as measured in accordance with the GPC procedure of ASTM D-3593-80, a melting point of about 67°C, and a Young's Modulus of about 400 kg/cm 2 , a dynamic puncture value of about 11.5 cmkg/mil (4528 cmkg/cm), and a Vicat softening point of about 62°C, as measured in accordance with ASTM D-1525. A film according to any one of claims 1 to 7 wherein the ethylene alpha olefin copolymer and/or said another ethylene alpha olefin copolymer is a butene-hexene terpolymer having a density of about 0.900 g/cm 3 , a melt index of about 1.2 g/10 min, an Mw value of about 92,000, an Mn value of about 50,000, a melting point of about 94°C, and a Young's Modulus of about 773 kg/cm 2 , and a Vicat softening point of 75 - 83°C as measured in accordance with ASTM D-1525. A film according to any one of claims 1 to 11 which is cross-linked, preferably by irradiation. A film according to any one of claims 1 to 12 which shrinks at least 55% in both the machine and transverse directions at 90°C. An irradiated film according to claim 10 comprising three layers including a vinylidene chloride-methyl acrylate copolymer oxygen barrier-core layer and an outer layer on each side of said barrier-core layer comprising a blend of between 65 and 75% of said ethylene alpha olefin copolymer and between 20 and 30% ethylene vinyl acetate, and said film shrinks at least 55% in both the machine and transverse directions at 90°C. A film according to claim 2 having a dynamic puncture strength of at least 10 cmkg/mil (3937 cmkg/cm). A film according to claim 14 having a dynamic puncture strength of at least 3 cmkg/mil (1181 cmkg/cm).

Inventors: GEORGELOS PAUL NICK, GEORGELOS, PAUL NICK
Applicants: VISKASE CORP, VISKASE CORPORATION
Application year: 1994
Main classification: C08J5
IPC codes: B29L7, B29C61, B32B27, C08L23, C08J5, B29K23, B29C55, B29L9